This invention relates to the contact operating mechanism of a circuit breaker and more particularly to a cam structure in that mechanism which improves blow-open performance of the contact arm of the circuit breaker during short circuit conditions.
The terms "blow open" or "blow off" are commonly used to describe a current interrupting mechanism which is used to handle very large short-circuit overcurrent conditions (e.g. when the current flow may be greater than 100 times the rated current of the breaker). The blow open mechanism causes the breaker contacts to open during the first millisecond that the overcurrent condition exists. This rapid operation is important to limit the current flow to a fraction of the available current and, therefore, to limit damage to the breaker and to apparatus connected to receive power through the circuit breaker.
The blow open force is a magnetic force which is generated by the large current flowing through a load contact arm (load blade) and a line contact arm (line strap) of the circuit breaker. To generate sufficient force to "blow open" the load and line contacts, the breaker is designed such that the load blade is in close proximity to and parallel to the line strap at least along part of its length. In addition, the currents flowing through the parallel portions of the load blade and the line strap are in opposite directions. This current flow produces opposing magnetic fields. Because the load blade and line strap are in close proximity, these opposing magnetic fields interact strongly, producing forces sufficient to blow the contacts apart more quickly than the current flow could be stopped by the instantaneous tripping function of the circuit breaker mechanism. When the contacts have been blown open, some current will continue to flow due to electrical arcs in the arc chamber and ionization of the air in the arc chamber. These currents plus the initial overcurrent condition, activate the trip mechanism of the breaker to ensure that the contacts do not reclose after they have been blown open.
The strength of the magnetic fields is a function of: 1) the amount of current flowing through the breaker, 2) the length of the parallel portions of the load blade and line strap and 3) the separation between the load blade and line contact. While this force can be made quite large by lengthening the parallel portions of the load blade and line strap, it may be difficult to implement a design of this type in the small space that is typically allowed for a circuit breaker. The blow-open force may also be increased by reducing the separation between the load blade and the line strap. This minimum separation, however, is limited by factors such as the need for strong electrical insulation between the load blade and line strap, the strength of the housing for the breaker and the ease with which the breaker may be assembled.
Another way in which the blow open force may be adjusted is to reduce the biasing force that holds the contacts closed during normal operation. If this force is reduced to too great an extent, however, the contacts may undesirably open during normal operation.
Some circuit breakers provide contact pressure by means of a plain spring-biasing the contact arm to the closed position. During blow open, the spring provides an opposing force that increases and is proportional to angle of opening of the contact arm. A problem with this structure is that the contact arm opens more slowly during a short circuit due to the higher opposing spring forces, and the contact arm is more likely to reclose before the electric current stops flowing.
A further conventional circuit breaker requires different amounts of force for normal opening and for a blow open condition. This capability is provided via a cam surface fixed to the crossbar, and a spring-biased pin that slides in a slot in the contact arm. A disadvantage of such a construction is that it requires a multi-piece crossbar because the cam needs to be metallic in order to resist wear. In other systems, this capability is provided by a cam surface on the edge of the contact arm. A spring-biased member acts against the cam-shaped edge of contact arm near the pivoting end. Such a structure typically requires a relatively large amount of space.
Still another conventional circuit breaker uses a spring, acting in compression, with one end hinged on a molded crossbar and the other end hinged on the contact arm. This creates a bi-stable toggle action. The disadvantages of this design, are (1) typically, the toggle mechanism is not compact because the spring must swing through a wide rotation angle relative to the crossbar, and (2) the toggle mechanism may cause a torque acting against the operating mechanism after a blow open event, reducing the force available to rotate the crossbar to the open position.
An improved circuit breaker is desired for quickly opening in a blow open condition, without occupying excessive space.